Once you unlock the toilet at Tavern level 3, you have to wait until one of your customers uses it. You can notice them exiting through your backdoor, follow them towards the toilet.

When they are on the toilet and have the door shut, simply stand at the door and look through the small opening. To be sure, you can keep knocking on the door until you get the -10 reputation for being rude to customers, that's when I got the achievement.

Good luck!

You unlock the toilet at Tavern level 3 (simply keep serving customers until you can buy that level):

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The toilet will be located to the left of the well. You will also be pointed towards it if you follow the quests:

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Knock on the door until you get the -10 reputation (top left corner):

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That's it! You should now have the achievement.